Depending upon the task to be performed, the degree of accuracy to which the position of a clamping face must be adjusted ranges from roughly approximate to extremely precise. An example of the former is a bench vise wherein the work piece is positioned only approximately and the location of the operation to be performed thereon is more closely determined by the vise user. An example of a work clamping tool used for this purpose is shown in U.S. Pat. No. 1,121,531. Examples of other work clampings devices which include an adjustment feature are shown in U.S. Pat. Nos. 1,666,079 and 2,494,424. While used for position adjustment rather than work clamping, another form of adjustment mechanism is shown in U.S. Pat. 2,027,888.
Certain industrial manufacturing processes require that clamping of a work piece be performed with somewhat greater precision than is possible with the common bench vise. In addition, such processes often require clamping of work pieces having random thicknesses. Sometimes such pieces must be clamped in one of several positions and/or from one side only, i.e., against the horizontal working surface of a bench. Devices known as step clamps, step blocks, toe clamps and gripper buttons are made for such purposes and are available from Monroe, Jergens, Applied Mechanics Corp. and Carr Lane Manufacturing Company. While clamping tools of these types provide somewhat more flexibility and precision than a common a bench vise, they are wholly unsuited for sophisticated, high production work clamping applications.
One of the most demanding work clamping applications arises in the automotive industry. The construction of vehicle bodies requires that two sheet metal body parts be clamped in precise relative location one to the other while they are being attached to one another by spot welding or carbon dioxide welding. Such precision alignment is necessary for several reasons. First, the overall appearance and saleability of the-vehicle is improved when its sheet metal body parts are carefully and precisely joined to one another. The customer is therefore likely to be much more satisfied with the appearance of the vehicle and to recommend a similar purchase to others.
Precision of sheet metal assembly is also required since the various cavities defined by the body shell must receive and have attached thereto a wide variety of carefully formed interior and exterior vehicle components. Dashboard assemblies, window regulators and seats are among those components requiring closely and accurately fitted sheet metal body parts in order to assure final assembly which is both workmanlike and of neat appearance.
Such automotive work clamping applications not only demand precision clamping but rapidity of adjustability is also of critical importance. Time spent in tool adjustment is non-productive and impairs the progress of high speed assembly lines. The cost per unit downtime in an auto assembly line is enormous and therefore is to be avoided if at all possible.
In the automotive industry, sheet metal body parts are usually clamped between a pedestal mounted stationary locator block and another locator block mounted on a movable clamp arm. The clamp arm, in turn, is supported by a hand clamping mechanism or by a hydraulic or pneumatic automatic clamping mechanism, the latter types often being found in high rate production applications.
A significant factor affecting the precision with which sheet metal parts are clamped for welding attachment is that such parts, especially larger, contoured parts, tend to vary slightly in size and shape and between successive set ups.
For each set-up run of sheet metal panels to be joined, the relative positions of the fixed and movable locator blocks must be quickly and precisely selected. This is so since there will be slight variances, set-up to set-up, in the shape of a given sheet metal part. Quick, precise positioning of locator blocks dramatically reduces downtime. In addition, the clamping arrangement must also secure the sheet metal parts for welding without marring or otherwise deforming the metal surface. To do otherwise may impair the quality and appearance of the exterior finish.
A common approach to the problem of precise locator block positioning is to form the structure of the block supporting pedestals using multiple pieces bolted together in one or more planes. By so doing, the precise location of the locator block may be adjusted by the insertion of shims, thin pieces of metal inserted between the interfaced surfaces of the structure which supports a particular locator block. For example, the stationary structure supporting a nest locator block is constructed to have a base and a locating nest which are bolted together and which are arranged to receive one or multiple shims between their interfaced surfaces. This permits selection of the precise final clamping position of the nest locator block. Similarly, the movable clamp locator block and the supporting clamp arm are bolted together and constructed to permit the interposition of shims whereby the final clamping position of the movable locator block may be precisely selected.
Clamping structures which support the locator blocks are manufactured in a wide variety of shapes and bolt hole patterns. In fact, a single welding fixture may employ clamping structures requiring shims of varying perimeter shapes and varying cut-out patterns to accommodate the wide variety of clamping structures and bolt hole patterns used therein. For these reasons, an automotive body assembly plant will be required to stock dozens if not hundreds of different sizes, shapes and thickness of shims for this purpose.
When an adjustment of the position of locator blocks is required to be made, the maintenance person must first assure that shims of the correct size, thickness and bolt pattern are on hand. To perform the actual adjustment, bolts are loosened on the clamping structure and shims are added or removed to both the stationary pedestal and to the movable clamping structure until both locator blocks are positioned as desired. The time required to perform such adjustments is very significant and the need to stock such a wide variety of shims presents an imposing inventory and purchasing burden.
Yet another disadvantage arising from such an arrangement is that because the adjustment procedure is relatively complicated, painstaking and time consuming, there is a significant chance that it will be performed incorrectly. Poorly assembled sheet metal body components and customer dissatisfaction will be the result. An adjustment mechanism for work clamping which is easy and quick to use, which is capable of accurately positioning locator blocks and which eliminates the need for adjustment shims would be a distinct advance in the art.